Field
This disclosure relates generally to the field of hydrated glass materials and specifically to a method of forming an optical quality article from such materials.
The manufacture of glass articles useful for fine optical applications and having good durability, light transmissivity, and surface smoothness commonly requires at least three basic steps. Firstly, a base glass is formed from selected ingredients via conventional glass forming techniques. Secondly, the glass surface is ground to achieve a relatively rough surface having an approximate curvature or degree of flatness. Lastly, that surface is subjected to a polishing step to remove surface imperfections such that, after polishing, there is obtained an optical quality surface smoothness. Typically, to assure surface smoothness of precision optical quality, the surface roughness (Roughness Height) of the glass should not exceed one-tenth of the wavelength of light being transmitted. Since the wavelengths of visible light range from about 16 microinches (16.mu.") to 30 microinches (30.mu."), this means that the surface roughness should not exceed about 1.6 microinches in the case of violet light and 3.0.mu." in the case of red light.
The relative surface smoothness (or low degrees of surface roughness) of an article can be precisely measured by known means. For example, in one method, the surface characteristics of an article are measured with an instrument which amplifies and displays the displacement of a stylus-like arm which is slowly drawn across the surface of the article. The results can be viewed on a chart as an actual reproduction of the surface being examined, amplified as high as 100,000 times. Alternatively, the results can be described in terms of "Roughness Height." This expression, for purposes of defining a standard, is described as the arithmetical average (AA) deviation expressed in microinches (.mu.") measured normal to the centerline. Arithmetic Average (AA) is also known in British Standards as Center Line Average (CLA). This terminology is explained more fully in the publication, Surface Texture, ASA B 46.1-- 1962, published by the American Society of Mechanical Engineers, New York, N.Y. Typical of the instruments which can measure fine degrees of surface smoothness (peaks and valleys) are instruments known commercially as a Proficorder or a Surfanalyzer. Detailed descriptions concerning the use of such instruments can be found in manuals used with the instruments and other publications. According to one such manual which accompanies the Gould Surfanalyzer, Model 1200, the results of surface measurement, expressed in AA, can be converted approximately to the root mean square (rms) average by multiplying the AA by 1.11. Hence, surface smoothness, or a low degree of surface roughness, can be expressed in AA units or rms units over a given surface. An example of rms measurements is described in an article entitled "Polishing of Supersmooth Metal Mirrors," Applied Optics, Vol. 14, No. 8, pp. 1808-1812, Aug. 1975. See also an article entitled "Surface Characterization: A Total Approach," Research/Development, Nov. 1975 which describes the use of stylus instruments.
As used herein, the expression "optical quality surface" or its equivalent, refers to a glass surface having a "Roughness Height" the AA of which is less than 3.0 microinches (3.0.mu.") over a surface distance of at least 0.1 inch. A preferred optical quality surface has a Roughness Height the AA of which is less than 1.6 microinches (1.6.mu.") over a surface distance of at least 0.1 inch, so that the entire range (16 to 30.mu.") of visible light can pass on or through the surface with minimal diffusion or scattering.
In addition to a high degree of surface smoothness, optical glass articles such as lenses should meet minimum standards for durability and light transmissivity. One common test for chemical durability of an optical glass lens is known as American Optical Stability Test 5.2.0.0.6. which measures durability by noting the mg/cm.sup.2 of glass lost after a defined exposure to a defined acid environment. An acceptable "standard" of durability is achieved if the amount of glass loss is less than 0.05 mg/cm.sup.2 under the test conditions.
Visible light transmissivity can be measured by determining the percentages of selected wavelengths of visible light (e.g. 350, 400, 500, and 700 nm) which pass through a tested glass article under defined conditions. A common method for determining such light transmission involves the use of visible spectroscopy.
The present invention is concerned with a method of making an optical glass article which has a high degree of surface smoothness durability and light transmissivity. The method is unique in that costly and time consuming grinding and polishing steps are not used.
Prior Art
Glass materials conventionally used in the preparation of fine optical articles cannot be readily molded by known molding techniques to achieve an optical quality surface. In recent years, however, it has been found that certain glass compositions can be successfully hydrated to achieve rubbery or plastic-like properties. Such glasses have become known as hydrated glasses because they include varying amounts of water within the glass. See, for example, U.S. Pat. No. 3,498,802 and U.S. Pat. No. 3,498,803 which disclose methods of including water within certain types of glass to impart properties not commonly associated with glass per se. See also U.S. Pat. No. 3,811,853 which discloses the hydration of such glasses under acidic conditions. More recently, in U.S. Pat. No. 3,912,481, issued in the names of R. Bartholomew et al., on Oct. 14, 1975, it has been disclosed that glass articles can be thermoplastically formed at relatively low temperatures by forming such glasses in a two-step process. In the first step, excess water is introduced into a base anhydrous glass. Then, the water content is reduced to a defined range via a partial dehydration step.
In preparing hydrated glasses, it has been noted that to facilitate hydration, a minimum amount of alkali must be present as a glass constituent. The presence of alkali also permits hydration with less water. Commonly however, the presence of such alkali (Na.sub.2 O and/or K.sub.2 O, for example) results in less than desired final durability of the hydrated product. To achieve an acceptable durability, it has been found that an acidic hydration step can be utilized to effectively reduce the total alkali content. However, in hydrating the glass under acidic conditions, it has been found that undesirable alkali gradients result in the glass (e.g., the alkali content varies from the surface to the center of the final article). Hence, this disadvantage has tended to nullify the overall advantages of an acid hydration step. I have now found a method of preparing a moldable glass article via an acidic hydration step without that disadvantage. Details of my method are described below.